A developing method wherein friction charging is generated by mixing and stirring electrophotographic printing-use toner (referred to as toner hereinafter) with magnetic powder carrier (referred to as carrier hereinafter) such as ferrite carrier, and the mixture of toner and carrier is held on a magnetic roller in a brush-shape so as to transfer the toner onto an electrostatic latent image formed on a photoreceptor by an electrostatic force, namely, the magnetic brush method has been widely known as one of the methods for developing an electrostatic image by toner.
Developer which is adopted in the magnetic brush method such as above is two-component developer composed of toner and carrier, wherein toner has a function of making an electrostatic latent image formed on the photoreceptor into a visible image, and carrier has a function of supplying charge to the toner by friction charging and transferring the toner onto a development region.
Toner is mostly made of binding agent synthetic resin (binding resin), and a charge control agent, coloring agent, a parting agent, and other agents are dispersed in the binding agent synthetic resin so as to form a toner mother particle, and a fluidizing agent is added onto the surface of the toner mother particle.
To explain briefly the function of the above materials constituting toner, (1) the binding agent synthetic resin gives binding, stability, and friction charging characteristics, (2) the coloring agent gives color and a friction charging characteristic, (3) the charge control agent gives a friction charging characteristic, (4) the parting agent gives a cleaning characteristic and prevents fixing offset, and (5) the fluidizing agent gives fluidity, friction charging, and cleaning characteristics.
Incidentally, in the magnetic brush method, a problem of so-called fogging in which toner is spread to a non-image region is one of the most important problems to be solved in order to realize a quality image. In particular, recently, in response to a long distance shipping of toner, for example, in exporting, a problem of fogging caused by exposure to high temperature during shipment has been increasing.
It is speculated that the fogging is generated in toner which is exposed to high temperature as a result of aggregation of toner which occurs at high temperature.
Namely, when a toner particle adheres to another toner particle by aggregation, each toner contacting with another toner particle lose a surface area for contacting with carrier by the area making a contact with another toner, and the charging ability of the toner by friction charging is deteriorated by the amount of the lost surface area. When toner whose charging ability is deteriorated by the aggregation is replenished into the developer tank, the toner is released out of the developer tank without being sufficiently charged by the friction with the carrier. The toner released out of the developer tank with insufficient charging is not properly guided to the image region on the photoreceptor since the electrostatic force, generated by the development electric field, acting on such toner is small. As a result, the toner adheres to the non-image region and causes the fog to generate.
Thus, it can be said that preventing the deterioration of charging ability caused by the aggregation of toner at high temperature is important in finding means to solve the problem of the fog phenomenon.
Conventionally, as such means for solving the fog phenomenon, a method for adding the fluidizing agent onto the surface of the toner mother particle in excess which is more than the amount added to obtain sufficient fluidity at normal temperature has been widely adopted. Specifically, a large amount of fluidizing agent exceeding 0.40 part by weight is added with respect to 100 parts by weight of toner mother particles.
However, in the above conventional method for preventing deterioration of charging ability caused by aggregation of toner at high temperature, by adding a large amount of fluidizing agent, the charging ability of the toner mother particles is greatly changed, and the following problem is presented.
That is, in the case of adding a large amount of fluidizing agent whose polarity is opposite to that of the toner mother particle, a large amount of fluidizing agent having the opposite polarity is included in the toner. The fluidizing agent having the opposite polarity lowers the charging ability of the toner, and causes fog or scattering of toner to generate regardless of the presence or absence of high temperature.
On the other hand, in the case of adding a large amount of fluidizing agent having the same polarity as that of the toner mother particle, the charging ability of the toner is increased too much, and lowering of image density is induced during running.